Recently, it is needed to measure very small concentrations in an aqueous solution in a semiconductor fabrication process or the like. For example, it is needed to measure and manage concentrations in an etching liquid or a cleaning liquid used in an etching process for a silicon wafer or the like precisely, simply and fast.
The inventors proposed a chemical analysis using far ultraviolet spectroscopy in order to analyze very small concentrations of solutes in an aqueous solution (JP-A 2005-214863, and Applied Spectroscopy Vol. 58 (2004)910-916). In the analysis, absorption of far ultraviolet light is measured in a slope (for example, 170 to 210 nm) in the higher wavelength side of a peak of an absorption band of water (due to n→σ* transition of water molecules) appearing in the far ultraviolet range. Because the absorption band is very sensitive to a change in hydrogen bonds of water molecules, a quantitative measurement can be performed on components hydrated in an aqueous solution with higher sensitivity than in near infrared and infrared spectroscopy. In some cases, absorption spectra of a water-soluble component itself also appear in the far ultraviolet range below 300 nm, and a plurality of soluble components can be analyzed with far ultraviolet spectroscopy in a wavelength range between 170 and 300 nm.
However, as the measurement wavelength of transmitting light becomes shorter further in the far ultraviolet range, optical absorption due to water becomes larger, and the transmittance becomes smaller. Therefore, a spectroscopic measurement becomes impossible if an optical cell with a very short optical path is not available. In order to solve this problem, the inventors focus attention to an attenuated total reflection optical probe (ATR probe). Light absorption due to attenuated total reflection is explained here. When a light ray entering a medium having a higher refractive index (such as synthetic quartz) is incident on an interface between the medium and another medium having a lower refractive index (for example, a sample to be measured such as water), the light ray is reflected totally if the incident angle is larger than the critical angle. However, the light ray penetrates into the other medium having a lower refractive index, by a certain distance of the order of wavelength, propagates in the direction of the interface, and is reflected. This penetrating light ray is called evanescent waves. The amplitude of electric field of the evanescent waves is highest at the reflection point, and it attenuates quickly in a direction perpendicular to the interface and along the interface. The distance at which the amplitude of electric field decreases to 1/e is called penetration depth. According to the attenuated total reflection spectroscopy, light is absorbed due to the penetration of the evanescent waves of the order of wavelength, and the light absorption can be detected in the reflected light. Because the penetration depth corresponds to optical path length in a conventional transmission spectra measurement, absorption spectra similar to that obtained with a very short optical path length can be realized theoretically.
It is to be noted that a material of an ATR probe is limited because it should have refractive index always higher than the sample and sufficient transmittance in the measurement wavelength range. Then, the inventors thought that a special type of ATR probe is necessary in order to measure the absorption band due to n→σ* transition in water in the far ultraviolet range because the above-mentioned conditions on the refractive index and transmittance have to be satisfied, and they proposed a special type of ATR probe (JP-A 2007-279025).
The invention can be applied to concentration measurement for a processing liquid used in a semiconductor process, and a prior art concentration measurement is explained here. As to a processing liquid of mixed acids used in silicon wafer cleaning process, photo-etching process and the like, a cleaning water having radical components such as hydroxyl radicals and the like, concentration management is necessary on viewpoints of yield, safety, working efficiency and the like, and concentration analysis is needed for the concentration management. Recently, various types of methods are proposed (for examples, JP-A 2007-155494, 2006-234663 and 7-12713). However, in these measurement methods, for example, a cleaning liquid overflowing from a processing vessel is sampled, or a liquid in a circulation line is sampled. Thus, they cannot be used to measure the concentration directly in real time. Recently, because etching and cleaning processes are controlled at higher precision, it is needed to monitor correct concentrations in a cleaning liquid in a processing vessel, and still further to measure concentration distribution in the processing liquid. In order to solve such a problem, a compact optical probe of immersion type is proposed (JP-A 2006-23200), and it can be used for an in-line measurement on temperature and solute concentrations at any point in a vessel. However, immersing a probe into a vessel has problems on an influence on circulation of cleaning liquid in the vessel, and on decrease in capacity for a wafer or wafers to be immersed.